Despite the strongly dispersive nature of guided wave propagation in isotropic planar waveguides, it is shown that material non‐linearity can result in the generation of propagating harmonics at points in the guided wave spectrum where the dispersion curves of all three orthogonal modes intersect. Experimental measurements of linear and harmonic guided wave propagation in rolled aluminum sheets have been explained in terms of the existence of a low symmetry microstructural texture in the materials. © 2003 American Institute of Physics